U.S. patent number 6,553,012 [Application Number 09/355,137] was granted by the patent office on 2003-04-22 for method and apparatus for directional radio communication.
This patent grant is currently assigned to Nokia Telecommunications Oy. Invention is credited to Marcos Katz.
United States Patent |
6,553,012 |
Katz |
April 22, 2003 |
Method and apparatus for directional radio communication
Abstract
A method of directional radio communication between a first
station (BTS4) and a second station (MS) comprises the following
steps. A plurality of consecutive signals are received at the first
station (BTS4) from the second station (MS). The signals are each
receivable from at least one of a plurality of different
directions. For each of a plurality of sequential signals which are
received by the first station (BTS4) from the second station (MS)
the value of at least one parameter is determined. The value of at
least one parameter is selected for a signal to be transmitted from
the first station (BTS4) to the second station (MS), the value of
the at least one parameter of the signal to be transmitted by the
first station (BTS4) being selected in dependence on the determined
values of the at least one parameter of the plurality of sequential
signals.
Inventors: |
Katz; Marcos (Oulu,
FI) |
Assignee: |
Nokia Telecommunications Oy
(Espoo, FI)
|
Family
ID: |
8166513 |
Appl.
No.: |
09/355,137 |
Filed: |
September 17, 1999 |
PCT
Filed: |
February 13, 1997 |
PCT No.: |
PCT/EP97/00665 |
PCT
Pub. No.: |
WO98/36597 |
PCT
Pub. Date: |
August 20, 1998 |
Current U.S.
Class: |
370/328;
342/373 |
Current CPC
Class: |
H04B
7/0491 (20130101); H04W 16/28 (20130101); H04B
7/0615 (20130101); H04B 7/0617 (20130101) |
Current International
Class: |
H04Q
7/36 (20060101); H04B 7/04 (20060101); H04B
7/06 (20060101); H04Q 007/00 () |
Field of
Search: |
;455/561,562,456,507,449,426,450 ;370/328,336,337,345,342
;342/373,372 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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647 978 |
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Apr 1995 |
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EP |
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715 478 |
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Jun 1996 |
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EP |
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729 285 |
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Aug 1996 |
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EP |
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755 090 |
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Jan 1997 |
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EP |
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755 093 |
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Jan 1997 |
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EP |
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96/00466 |
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Jan 1996 |
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WO |
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96/09696 |
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Mar 1996 |
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WO |
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WO 96/37969 |
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Nov 1996 |
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WO |
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Primary Examiner: Vo; Nguyen T.
Assistant Examiner: Ly; Nghi H.
Attorney, Agent or Firm: Merchant & Gould P.C.
Claims
What is claimed is:
1. A method of directional radio communication between a first
station and a second station, said method comprising the steps of:
receiving at said first station a plurality of consecutive signals
from said second station, said signals each being receivable from
at least one of a plurality of different directions; determining a
value of at least one parameter for each of a plurality of
sequential signals from the consecutive signals which are received
by the first station from the second station; and selecting a value
of at least one parameter for a signal to be transmitted from said
first station to said second station, said value of the at least
one parameter of the signal to be transmitted by the first station
being selected in dependence on said determined value of said at
least one parameter of said plurality of sequential signals,
wherein said selecting step comprises applying a weighting pattern
to said plurality of sequential signals.
2. A method as claimed in claim 1, wherein said step of determining
a value of at least one parameter comprises determining the or each
direction for each of the plurality of sequential signals from
which said sequential signals are received and said selecting step
comprises selecting at least one direction for the transmission of
a signal from the first station to the second station, said at
least one direction being selected in dependence on the determined
direction for said plurality of sequential signals.
3. A method as claimed in claim 1, wherein said step of determining
a value of at least one parameter comprises determining the
strength of each of said plurality of sequential signals and said
selecting step comprises selecting the strength of the signal to be
transmitted to said second station, the strength of said signal
being selected in dependence on said determined strengths for said
plurality of sequential signals.
4. A method as claimed in claim 1, wherein said weighting pattern
is a uniform weighting pattern, so that each of the sequential
signals is given equal weight.
5. A method as claimed in claim 1, wherein said weighting pattern
is such that the more recently received ones of said plurality of
sequential signals are given more weight than the less recently
received ones of said plurality of sequential signals.
6. A method as claimed in claim 5, wherein said weighting pattern
is an exponential or a linearly increasing weighting pattern.
7. A method according to claim 1, including the step of determining
the radio environment.
8. A method as claimed in claim 7, wherein said selecting means
selects one of a plurality of weighting patterns in dependence on
the radio environment.
9. A method as claimed in claim 1, comprising the step of defining
at the first station a plurality of beam directions for
transmitting a radiation beam, wherein each of said beam directions
is individually selectable.
10. A method as claimed in claim 1, wherein said first station is a
base transceiver station in a cellular network.
11. A method as claimed in claim 1, wherein said second station is
a mobile station.
12. A first station for directional radio communication with a
second station, said first station comprising: a receiver for
receiving a plurality of consecutive signals from said second
station, said signals each being receivable from at least one of a
plurality of different directions; circuitry for determining the
value of at least one parameter for each of a plurality of
sequential signals from the consecutive signals which are received
by the first station from the second station; a transmitter for
transmitting a signal from the first station to the second station;
and a controller for controlling said transmitter, said controller
being arranged to select a value of at least one parameter for the
signal to be transmitted by said transmitter, said value of at
least one parameter being selected in dependence on the determined
values of said at least one parameter for said plurality of
sequential signals, wherein said controller is arranged to apply a
weighting pattern to said plurality of sequential signals.
13. A first station as claimed in claim 12, wherein said circuitry
for determining the value of at least one parameter is arranged to
determine the or each direction for each of the plurality of
sequential signals and the controller is arranged to select at
least one direction for the transmission of the signal by the
transmitter, said at least one direction being selected in
dependence on the determined directions for said plurality of
sequential signals.
14. A first station as claimed in claim 12, wherein said circuitry
for determining the value of at least one parameter is arranged to
determine the strength of each of said plurality of sequential
signals and the controller is arranged to select the strength of
the signal to be transmitted by the transmitter, the strength of
the signal being selected in dependence on the determined strengths
for said plurality of sequential signals.
15. A first station as claimed in claim 12, wherein a store is
provided for storing said determined parameters for each of the
plurality of sequential signals.
Description
The present invention relates to a method and apparatus for
directional radio communication in which signals between a first
station and a second station may be transmitted only in certain
directions. In particular, but not exclusively, the present
invention is applicable to cellular communication networks using
space division multiple access.
With currently implemented cellular communication networks, a base
transceiver station (BTS) is provided which transmits signals
intended for a given mobile station (MS), which may be a mobile
telephone, throughout a cell or cell sector served by that base
transceiver station. However, space division multiple access (SDMA)
systems have now been proposed. In a space division multiple access
system, the base transceiver station will not transmit signals
intended for a given mobile station throughout the cell or cell
sector but will only transmit the signal in the beam direction from
which a signal from the mobile station is received. SDMA systems
may also permit the base transceiver station to determine the
direction from which signals from the mobile station are
received.
SDMA systems may allow a number of advantages over existing systems
to be achieved. In particular, as the beam which is transmitted by
the BTS may only be transmitted in a particular direction and
accordingly may be relatively narrow, the power of the transceiver
can be concentrated into that narrow beam. It is believed that this
results in a better signal to noise ratio with both the signals
transmitted from the base transceiver station and the signals
received by the base transceiver station. Additionally, as a result
of the directionality of the base transceiver station, an
improvement in the signal to interference ratio of the signal
received by the base transceiver station can be achieved.
Furthermore, in the transmitting direction, the directionality of
the BTS allows energy to be concentrated into a narrow beam so that
the signal transmitted by the BTS can reach far away located mobile
stations with lower power levels than required by a conventional
BTS. This may allow mobile stations to operate successfully at
greater distances from the base transceiver station which in turn
means that the size of each cell or cell sector of the cellular
network can be increased. As a consequence of the larger cell size,
the number of base stations which are required can also be reduced
leading to lower network costs. SDMA systems generally require a
number of antenna elements in order to achieve the required
plurality of different beam directions in which signals can be
transmitted and received. The provision of a plurality of antenna
elements increases the sensitivity of the BTS to received signals.
This means that larger cell sizes do not adversely affect the
reception of signals by the BTS from mobile stations.
SDMA systems may also increase the capacity of the system, that is
the number of mobile stations which can be simultaneously supported
by the system is increased. This is due to the directional nature
of the communication which means that the BTS will pick up less
interference from mobile stations in other cells using the same
frequency. The BTS will generate less interference to other mobile
stations in other cells using the same frequency when communicating
with a given MS in the associated cell.
Ultimately, it is believed that SDMA systems will allow the same
frequency to be used simultaneously to transmit to two or even more
different mobile stations which are arranged at different locations
within the same cell. This can lead to a significant increase in
the amount of traffic which can be carried by cellular
networks.
SDMA systems can be implemented in analogue and digital cellular
networks and may be incorporated in the various existing standards
such as GSM, DCS 1800, TACS, AMPS and NMT. SDMA systems can also be
used in conjunction with other existing multiple access techniques
such as time division multiple access (TDMA), code division
multiple access (CDMA) and frequency division multiple access
(FDMA) techniques.
One problem with SDMA systems is that the direction in which
signals should be transmitted to a mobile station needs to be
determined. In certain circumstances, a relatively narrow beam will
be used to send a signal from a base transceiver station to a
mobile station. Therefore, the direction of that mobile station
needs to be assessed reasonably accurately. As is known, a signal
from a mobile station will generally follow several paths to the
BTS. Those plurality of paths are generally referred to as
multipaths. A given signal which is transmitted by the mobile
station may then be received by the base transceiver station from
more than one direction due to these multipath effects.
In general, the decision as to the beam direction which is to be
used by the BTS in order to transmit a signal to a mobile station
is based on information corresponding to the data burst previously
received by the BTS from the given MS. As the decision is based on
information received corresponding to only one burst, problems may
occur if, for example, the data burst transmitted by the mobile
station is superimposed with strong interference.
An additional problem is that the direction in which a signal is to
be transmitted by the BTS to the mobile station is determined on
the basis of the uplink signals received by the BTS from the mobile
station. However, the frequencies of the down link signals
transmitted from the mobile station to the BTS are different from
the frequencies used for the signals transmitted by the BTS to the
mobile station. The difference in the frequencies used in the
uplink and downlink signals means that the behaviour of the channel
in the uplink direction may be different from the behaviour of the
channel in the downlink direction. Thus the optimum direction
determined for the uplink signals will not always be the optimum
direction for the downlink signals.
A method of transmitting a pilot signal in a code division multiple
access cellular radio system is disclosed in WO 96/37969. The
method involves receiving at a first station a plurality of signals
from a second station, determining a value of a parameter for each
received signal and selecting the value of a parameter for a signal
to be transmitted in dependence on the value of the parameter of
the received signals. This method searches from the best signal
continuously and determines the nature of the radio environment by
means of a plurality of phasing means. A method of directional
radio communication based on similar principles is disclosed in
U.S. Pat. No. 5,515,378.
It is therefore an aim of certain embodiments of the present
invention to address these difficulties.
According to a first aspect of the present invention, there is
provided a method of directional radio communication between a
first station and a second station, said method comprising the
steps of: receiving at said first station a plurality of
consecutive signals from said second station, said signals each
being receivable from at least one of a plurality of different
directions; determining a value of at least one parameter for each
of a plurality of sequential signals from the consecutive signals
which are received by the first station from the second station;
and selecting a value of at least one parameter for a signal to be
transmitted from said first station to said second station, said
value of the at least one parameter of the signal to be transmitted
by the first station being selected in dependence on said
determined value of said at least one parameter of said plurality
of sequential signals, wherein said selecting step comprises
applying a weighting pattern to said plurality of sequential
signals.
By basing a parameter of the signal to be transmitted from the
second station to the first station on the parameter of a plurality
of signals previously received by the first station, the problems
caused by, for example, strong interference in the most recently
received signal can be reduced.
Preferably, said step of determining a value of at least one
parameter comprises determining the or each direction for each of
the plurality of sequential signals and said selecting step
comprises selecting at least one direction for the transmission of
a signal from the first station to the second station, said at
least one direction being selected in dependence on the determined
directions for said plurality of sequential signals. By basing the
or each direction in which a signal is to be transmitted by the
first station to the second station on a plurality of signals
received from the second station, the probability that a signal
transmitted by the first station will be received by the second
station is increased.
Alternatively and/or additionally said step of determining a value
of at least one parameter comprises determining the strength of
each of said plurality of sequential signals and said selecting
step comprises selecting the strength of the signal to be
transmitted to said second station, the strength of said signal
being selected in dependence on said determined strengths for said
plurality of sequential signals. By basing the strength of the
signal to be transmitted by the first station to the second station
on a plurality of signals received from the second station, the
probability that the signal strength will be at the right level is
increased. If the signal strength is too low, the second station
may not receive the signal, whilst if the signal strength is too
high, the risk of interference is unnecessarily increased. It
should be appreciated that in those embodiments of the invention,
where the signal is transmitted in a plurality of different
directions, the strength of the signal in those different
directions may differ.
Preferably, the selecting step comprises applying a weighting
pattern to said plurality of sequential signals. The term weighting
pattern also includes algorithms which provide a weighting
function. The weighting pattern may be a uniform weighting pattern
so that each of said plurality of sequential signals is given equal
weight. The weighting pattern alternatively may be such that the
more recently received ones of said plurality of sequential signals
are given more weight than the less recently received ones of said
plurality of sequential signals. The weighting pattern may be an
exponential or linearly increasing weighting pattern. These are
just two examples of possible patterns. Any other suitable pattern
can be used. Alternatively, the weighting pattern may be defined by
an algorithm. It should be appreciated that in some embodiments the
weighting pattern is applied to values determined for said
parameter.
Preferably, the selecting means selects one of a plurality of
weighting patterns in dependence on the radio environment. The
weighting patterns may be as outlined previously. For example, in
static or slowly changing radio environments, the uniform weighting
pattern may be used since it can be expected that the determined
the or each direction or strength of the plurality of sequential
signals will remain generally the same for those plurality of
consecutive signals. Alternatively, if the radio environment is a
fast changing radio environment, then the linearly increasing or
exponential weighting pattern can be used. With these latter
patterns, the previously determined the or each direction or
strength of the plurality of received sequential signals will have
a negligible influence on the selected beam direction.
Preferably, the method further comprises the step of defining at
the first station a plurality of beam directions for transmitting a
radiation beam, wherein each of said beam directions is
individually selectable.
According to the second aspect of the present invention, there is
provided a first station for directional radio communication with a
second station, said first station comprising: receiver means for
receiving a plurality of consecutive signals from said second
station, said signals each being receivable from at last one of a
plurality of different directions; determining means for
determining the value of at least one parameter for each of a
plurality of sequential signals from the consecutive signals which
are received by the first station from the second station;
transmitter means for transmitting a signal from the first station
to the second station; and control means for controlling said
transmitter means, said control means being arranged to select a
value of at least one parameter for the signal to be transmitted by
said transmitter means, said value of at least one parameter being
selected in dependence on the determined values of said at least
one parameter for said plurality of sequential signals, wherein
said control means is arranged to apply a weighting pattern to said
plurality of sequential signals.
Preferably, said determining means is arranged to determine the or
each direction for each of the plurality of sequential signals and
the control means is arranged to select at least one direction for
the transmission of the signal by the transmitter means, said at
least one direction being selected in dependence on the determined
the or each direction for said plurality of sequential signals.
Alternatively or additionally said determining means is arranged to
determine the strength of each of said plurality of sequential
signals and the control means is arranged to select the strength of
the signal to be transmitted by the transmitter means, the strength
of the signal being selected in dependence on the determined
strength for said plurality of sequential signals.
The control means may be arranged to apply a weighting pattern to
said plurality of sequential signals.
Storage means may be provided for storing said determined
parameters for each of the plurality of sequential signals.
The receiver means and the transmitter means may comprise an
antenna array which is arranged to provide a plurality of signal
beams in a plurality of different directions. The antenna array may
comprise a phased antenna array or may comprise a plurality of
separate antenna elements each of which is arranged to provide a
beam in a defined direction. Two separate arrays may be provided,
one to receive signals and the other to transmit signals.
Alternatively, a single array may be provided both to receive and
to transmit signals.
The transmitter means may be arranged to provide a radiation beam
in a plurality of beam directions, wherein each of the beam
directions is individually selectable. Preferably, the strength of
each of the beam directions is individually selectable.
The present invention is particularly applicable to cellular
communication networks. In such networks the first station may be a
base transceiver station and the second station may be a mobile
station respectively. However, it should be appreciated that
embodiments of the invention may be applicable to any other type of
radio communication network where both the first and second
stations may be both stationary or both mobile.
For a better understanding of the present invention and as to how
the same may be carried into effect, reference will now be made by
way of example to the accompanying drawings in which:
FIG. 1 shows a schematic view of a base transceiver station (BTS)
and its associated cells sectors;
FIG. 2 shows a simplified representation of an antenna array and
the base transceiver station;
FIG. 3 shows the fixed beam pattern provided by the antenna array
of FIG. 2;
FIG. 4 shows a schematic view of part of a beam selection portion
of the digital signal processor;
FIG. 5 shows four different weighting patterns;
FIG. 6 shows a schematic view of the digital signal processor of
FIG. 2;
FIG. 7 illustrates the channel impulse response for four channels,
out of the eight channels; and
FIGS. 8a to 8c respectively illustrate the pattern selection data
stored by the memory 1, a weighting pattern stored by the spatio
temporal weighting pattern block and the data calculated by that
block.
Reference will first be made to FIG. 1 in which three cell sectors
2 defining a cell 3 of a cellular mobile telephone network are
shown. The three cell sectors 2 are served by respective base
transceiver stations (BTS) 4. Three separate base transceiver
stations 4 are provided at the same location. Each BTS 4 has a
separate transceiver which transmits and receives signals to and
from a respective one of the three cell sectors 2. Thus, one
dedicated base transceiver station is provided for each cell sector
2. The BTS 4 is thus able to communicate with mobile stations (MS)
such as mobile telephones which are located in respective cell
sector 2.
The present embodiment is described in the context of a GSM (Global
System for Mobile Communications) network. In the GSM system, a
frequency/time division multiple access F/TDMA system is used. Data
is trasmitted between the BTS 4 and the MS in bursts. The data
bursts include a training sequence which is a known sequence of
data. The purpose of the training sequence will be described
hereinafter. Each data burst is transmitted in a given frequency
band in a predetermined time slot in that frequency band. The use
of a directional antenna array allows space division multiple
access also to be achieved. Thus, in embodiments of the present
invention, each data burst will be transmitted in a given frequency
band, in a given time slot, and in a given direction. An associated
channel can be defined for a given data burst transmitted in the
given frequency, in the given time slot, and in the given
direction. As will be discussed in more detail hereinafter, in some
embodiments of the present invention, the same data burst is
transmitted in the same frequency band, in the same time slot but
in two different directions.
FIG. 2 shows a schematic view of one antenna array 6 of one BTS 4
which acts as a transceiver. It should be appreciated that the
array 6 shown in FIG. 2 only serves one of the three cell sectors 2
shown in FIG. 1. Another two antenna arrays 6 are provided to serve
the other two cell sectors 2. The antenna array 6 has eight antenna
elements a.sub.1 . . . a.sub.8. The elements a.sub.1 . . . a.sub.8
are arranged to have a spacing of a half wavelength between each
antenna element a.sub.1 . . . a.sub.8 and are arranged in a
horizontal row in a straight line. Each antenna element a.sub.1 . .
. a.sub.8 is arranged to transmit and receive signals and can have
any suitable construction. Each antenna element a.sub.1 . . .
a.sub.8 may be a dipole antenna, a patch antenna or any other
suitable antenna. The eight antenna elements a.sub.1 . . . a.sub.8
together define a phased array antenna 6.
As is known, each antenna element a.sub.1 . . . a.sub.8 of the
phased array antenna 6 is supplied with the same signal to be
trasmitted to a mobile station MS. However, the phases of the
signals supplied to the respective antenna elements a.sub.1 . . .
a.sub.8 are shifted with respect to each other. The differences in
the phase relationship between the signals supplied to the
respective antenna elements a.sub.1 . . . a.sub.8 gives rise to a
directional radiation pattern. Thus, a signal from the BTS 4 may
only be transmitted in certain directions in the cell sector 2
associated with the array 6. The directional radiation pattern
achieved by the array 6 is a consequence of constructive and
destructive interference which arises between the signals which are
phase shifted with respect to each other and transmitted by each
antenna element a.sub.1 . . . a.sub.8. In this regard, reference is
made to FIG. 3 which illustrates the directional radiation pattern
which is achieved with the antenna array 6. The antenna array 6 can
be controlled to provide a beam b.sub.1 . . . b.sub.8 in any one of
the eight directions illustrated in FIG. 3. For example, the
antenna array 6 could be controlled to transmit a signal to a MS
only in the direction of beam b.sub.5 or only in the direction of
beam b.sub.6. As will be discussed in further detail hereinafter,
it is possible also to control the antenna array 6 to transmit a
signal in more than one beam direction at the same time. For
example, a signal may be transmitted in the two directions defined
by beam b.sub.5 and beam b.sub.6. FIG. 3 is only a schematic
representation of the eight possible beam directions which can be
achieved with the antenna array 6. In practice, however, there will
in fact be an overlap between adjacent beams to ensure that all of
the cell sector 2 is served by the antenna array 6.
The relative phase of the signal provided at each antenna element
a.sub.1 . . . a.sub.8 is controlled by Butler matrix circuitry 8 so
that a signal can be transmitted in the desired beam direction or
directions. The Butler matrix circuitry 8 thus provides a phase
shifting function. The Butler matrix circuitry 8 has eight inputs
10a-h from the BTS 4 and eight outputs, one to each antenna element
a.sub.1 . . . a.sub.8. The signals received by the respective
inputs 10a-h comprise the data bursts to be transmitted. Each of
the eight inputs 10a-h represents the beam direction in which a
given data burst could be transmitted. For example, when the Butler
matrix circuitry 8 receives a signal on the first input 10a, the
Butler matrix circuitry 8 applies the signal provided on input 10a
to each of the antenna elements a.sub.1 . . . a.sub.8 with the
required phase differences to cause beam b.sub.1 to be produced so
that the data burst is transmitted in the direction of beam
b.sub.1. Likewise, a signal provided on input 10b causes a beam in
the direction of beam b.sub.2 to be produced and so on.
As already discussed, the antenna elements a.sub.1 . . . a.sub.8 of
the antenna array 6 receive signals from a MS as well as transmit
signals to a MS. A signal transmitted by a MS will generally be
received by each of the eight antenna elements a.sub.1 . . .
a.sub.8. However, there will be a phase difference between each of
the signals received by the respective antenna elements a.sub.1 . .
. a.sub.8. The Butler matrix circuitry 8 is therefore able to
determine from the relative phases of the signals received by the
respective antenna elements a.sub.1 . . . a.sub.8 the beam
direction from which the signal has been received. The Butler
matrix circuitry 8 thus has eight inputs, one from each of the
antenna elements a.sub.1 . . . a.sub.8 for the signal received by
each antenna element. The Butler matrix circuitry 8 also has eight
outputs 14a-h. Each of the outputs 14a to 14h corresponds to a
particular beam direction from which a given data burst could be
received. For example, if the antenna array 6 receives a signal
from a MS from the direction of beam b.sub.1, then the Butler
matrix circuitry 8 will output the received signal on output 14a. A
received signal from the direction of beam b.sub.2 will cause the
received signal to be output from the Butler matrix circuitry 8 on
output 14b and so on. In summary, the Butler matrix circuitry 8
will receive on the antenna elements a.sub.1 . . . a.sub.8 eight
versions of the same signal which are phase shifted with respect to
one another. From the relative phase shifts, the Butler matrix
circuitry 8 determines the direction from which the received signal
has been received and outputs a signal on a given output 14a-h in
dependence on the direction from which the signal has been
received.
It should be appreciated that in some environments, a single signal
or data burst from a MS may appear to come from more than one beam
direction due to reflection of the signal whilst it travels between
the MS and the BTS 4, provided that the reflections have a
relatively wide angular spread. The Butler matrix circuitry 8 will
provide a signal on each output 14a-h corresponding to each of the
beam directions from which a given signal or data burst appears to
come. Thus, the same data burst may be provided on more than one
output 14a-h of the Butler matrix circuitry 8. However, the signals
on the respective outputs 14a-h may be time delayed with respect to
each other.
Each output 14a-h of the Butler matrix circuitry 8 is connected to
the input of a respective amplifier 16 which amplifies the received
signal. One amplifier 16 is provided for each output 14a-h of the
Butler matrix circuitry 8. The amplified signal is then processed
by a respective processor 18 which manipulates the amplified signal
to reduce the frequency of the received signal to the baseband
frequency so that the signal can be processed by the BTS 4. To
achieve this, the processor 18 removes the carrier frequency
component from the input signal. Again, one processor 18 is
provided for each output 14a-h of the Butler matrix circuitry 8.
The received signal, which is in analogue form, is then converted
into a digital signal by an analogue to digital (A/D) converter 20.
Eight A/D converters 20 are provided, one for each output 14a-h of
the Butler matrix circuitry 8. The digital signal is then input to
a digital signal processor 21 via a respective input 19a-h for
further processing.
The digital signal processor 21 also has eight outputs 22a-h, each
of which outputs a digital signal which represents the signal which
is to be transmitted to a given MS. The output 22a-h selected
represents the beam direction in which the signal is to be
transmitted. That digital signal is converted to an analogue signal
by a digital to analogue (D/A) converter 23. One digital to
analogue converter 23 is provided for each output 22a-h of the
digital signal processor 21. The analogue signal is then processed
by processor 24 which is a modulator which modulates onto the
carrier frequency the analogue signal to be transmitted. Prior to
the processing of the signal by the processor 24, the signal is at
the baseband frequency. The resulting signal is then amplified by
an amplifier 26 and passed to the respective input 10a-h of the
Butler matrix circuitry 8. A processor 24 and an amplifier 26 are
provided for each output 22a-h of the digital signal processor
21.
Reference will now be made to FIG. 4 which shows a schematic
representation of a beam selection arrangement 101 which forms part
of the digital signal processor 21 and which is arranged to select
the beam direction or directions for a data burst to be transmitted
to a given mobile station by the BTS 4. The beam selection
arrangement 101 comprises a beam pre-selection block 100. The beam
pre-selection block 100 determines for each data burst received
from a given mobile station the or each direction from which that
given data burst is received. The beam pre-selection block 101 may
also determine the strength of the data burst received in each of
the different directions. The beam pre-selection block 101 may be
arranged to select only a limited number of beam directions as
representative of the direction from which a given data burst is
received. For example, the beam pre-selection block 100 may select
a single beam direction for each data burst. That single beam
direction may correspond to the direction from which the strongest
signal is received or may correspond to the direction from which
the data burst is received with minimum delay. Of course two or
more beam directions may be selected as representative of the
directions from which a given data burst is received. Any suitable
criteria can be used in order to make the beam selection. In some
embodiments of the invention, the beam selection criteria may be
varied for example to take into account changes in the radio
environment or changes in the distance between the mobile station
and the base transceiver station.
The beam direction or directions determined by the beam
pre-selection block 100 is output to a memory 102 via outputs 103.
Additionally, information on the strength of the data burst
received in the respective beam direction may also be output to the
memory 102. The memory 102 is arranged to store this information
for the n preceding burst received by the BTS from the desired MS.
The memory 102 may be in the form of a FIFO register. Thus, when
the beam direction and signal strength information for the next
data burst received from the desired MS is determined, that
information is stored in the memory 102 and the information on the
n+1 previous data burst is removed from the memory. In other words,
the oldest information in the register is shifted out in order to
make room for the newest information. Reference is made to FIG. 8a
which illustrates an example of the information stored in the
memory 102. For each beam, a one or a zero is stored for a given
data burst. A one indicates that the beam preselection block 100
selected that beam in the given data burst. In the example shown in
FIG. 8a, three beams are selected by the beam preselection block
100 for a given data burst. For example, for the ith data burst the
third, fourth and fifth beams were selected whilst the first,
second, sixth, seventh and eight beams were not selected.
The memory 102 has a plurality of outputs 104 which are connected
to a spatio-temporal weighting pattern block 106. In one embodiment
of the present invention, outputs 104 are provided. Thus, one
output 104 is provided for the information stored in the memory 102
for each one of the antenna elements. The spatio-temporal weighting
pattern block 106 applies a weighting pattern to the information
stored in respect of the K preceding data bursts received from the
given MS.
Reference is made to FIG. 8b which illustrates an example of a
weighting pattern stored by block 106. As can be seen from this
figure, the weighting pattern can be regarded as eight separate
patterns, one for each beam. Furthermore, as can be seen, the
weighting pattern differs for different beam directions. For
example, the weighting pattern for the first beam would be
(0,0,0,0) and (1,1,1,1) for the second beam. In this example, the
weighting pattern is only applied to the four preceding data
bursts. Thus K=4. In this example, the weighting pattern applied
constant weights to each of the four previously received data
bursts for each beam although the weighting patterns differs for
each beam.
Reference is now made to FIG. 5 which shows four examples of
weighting patterns. However it should be appreciated that these
four patterns are by of example only. In particular any suitable
weighting pattern or algorithm can be used.
FIG. 5a shows a first weighting pattern in which all the K
previously beam selections for previously received data bursts are
of equal importance in determining the beam direction or directions
in which the next data burst is to be transmitted by the BTS 4 to
the desired MS. FIG. 5b shows a second weighting pattern where the
closer in time that a previously received data burst is to the
currently received data burst, the higher its influence on the beam
direction or direction selected for the transmission of the next
data burst to the desired MS. In other words, the beam selections
made on the Kth previous burst will have a smaller influence on the
decision as to the direction in which the next burst will be
transmitted by the BTS to the desired MS as compared to the most
recently received data burst. The weighting pattern illustrated in
FIG. 5b shows a linear increasing pattern.
The weighting pattern shown in FIG. 5c is similar to that shown in
FIG. 5b in that the information on the Kth previous beam selections
made by the BTS has a small impact on the decision as to the
direction in which the next data burst is to be transmitted by the
BTS to the desired MS. The exponential increasing weighting pattern
shown in FIG. 5c thus takes only into account the information
determined by the beam pre-selection block 100 for only the most
recently received data bursts.
FIG. 5d shows the limit situation where the selection as to the
beam direction or directions in which a data burst is to be
transmitted by the BTS to the desired MS is based purely on the
data burst most recently received by the BTS from the desired MS.
This would be selected when the radio environment changes very
rapidly so that the previous beam selections would have very little
influence on the current beam selection.
The spatio temporal weighting pattern block 106 receives an input
from a weight pattern bank block 108. This block stores for example
the four patterns illustrated in FIG. 5. The weight pattern bank
block may also store any other suitable weighting pattern or
weighting pattern algorithm. In the latter case, the weighting
pattern need not follow a specific function. One or more weighting
pattern algorithms can be provided to replace the stored weighting
patterns. Any suitable number of weighting patterns can be stored.
For example in one embodiment of the invention, only a single
weighting pattern or algorithm defining a weighting pattern is
stored.
The weight pattern bank block 108 may also be arranged to select
the most appropriate weighting pattern for the current radio
environment and to output that weighting pattern to the
spatio-temporal weighting pattern block 106. When the radio
environment is static or slowly changing, then the pattern shown in
FIG. 5a would be selected. Thus, all of the K previous bursts are
all of equal importance in determining the direction in which the
next data burst is to be transmitted by the BTS. The impact
therefore of a data burst with, for example, a large amount of
interference is greatly reduced by the averaging effect of the
consideration of the K preceding data bursts.
In a fast changing radio environment, the pattern illustrated in
FIG. 5b or FIG. 5c could be used. A rapidly changing radio
environment may occur when a mobile station is moving quickly
through an urban environment. In such an environment, the more
recently received data bursts will be of more relevance than the
less recently received data bursts. Accordingly information on the
more recently received data bursts should have a greater influence
on the direction in which the or each direction in which the next
data burst is to be transmitted by the BTS 4.
In order to determine which weighting pattern is most appropriate,
information on the radio environment is input to the weight pattern
bank block 108. In one embodiment of the invention, the information
on the radio environment can be independently determined. However
in the preferred embodiment of the invention, the received data
bursts are used to provide the information on the radio
environment. For example, the weight pattern bank block 108 may
consider the direction or directions from which L preceding bursts
are received by the BTS from the desired MS. L may be the same or
different to K. If there is little change in the direction or
directions from which consecutive data bursts are received, then it
can be assumed that the radio environment is static or only slowly
changing. The weighting pattern is shown in FIG. 5a could be
selected. In contrast, when it is determined that the preceding L
data bursts are received from a relatively large number of
different directions, then it can be assumed that the mobile is in
a fast changing radio environment and either the pattern shown in
FIG. 5b or c can be selected. Information on the radio environment
may alternatively be prestored. In one further alternative, the
statistical information stored by the memory 102 may be used to
determine the type of radio environment.
The spatio temporal weighting pattern block 106 applies the
selected weighting pattern to the information provided by the beam
pre-selection block 101 for the K preceding data bursts received by
the BTS from the desired MS. The spatio temporal weighting pattern
block 106 calculates for each of the eight possible beam directions
a beam score using the following equation:
k=1,2 . . . 8
s(i,k) represents the score that the kth beam gets when deciding
the beam direction or directions which will be selected for
transmitting the ith (or next) burst from the BTS 4 to the desired
mobile station.
W.sub.ST (i,k) is a K.times.1 vector containing the K spatio
temporal real weights for the kth beam at the ith burst. This takes
into account the weighting pattern applied to the received beam
selection information for the K preceding bursts.
b.sub.ST (i,k) is K.times.1 binary vector containing statistical
information regarding the previous K selections of the Kth beam. If
the b.sub.ST (i,r)=1 where r=1-(K-1), . . . , i, this means that
the kth beam was selected by the beam pre-selection block 100
during the rth burst. If the b.sub.ST (i,r)=0, this means that the
kth beam was not selected during the rth burst. Accordingly, the
score s(i,k) will be 0 if the kth beam was not selected by the beam
pre-selection block for the K previous data bursts received from
the mobile station.
Reference is now made to FIG. 8c which shows the score vectors
calculated by the spatio temporal weighting pattern block 106 for
each of the eight beams using the selected weighting pattern. For
example the vector score for the third beam using the weighting
pattern shown in FIG. 8b is as follows: ##EQU1##
The spatio temporal weighting pattern has eight outputs 110 which
are input to the final beam selection block 112. Each of the
outputs 110 corresponds to one of the eight possible beam
directions. In one preferred embodiment of the invention, the score
vector calculated by the spatio temporal weighting pattern block
106 is input via the eight outputs 110 to the final beam selection
block 112. The final beam selection block 112 then selects those
beam directions will be used for transmitting the next data burst
from the BTS to the given MS. If only one beam direction is to be
used, the beam direction with the highest score out of the eight
possible computed scores is selected as the transmitting beam
direction. In the example illustrated in FIG. 8c, if only one beam
is to be selected, the third beam which has the highest score would
be selected. If three beams are to be selected, the third, fourth
and fifth beams would be selected. However, it will be appreciated
that any suitable number of beam directions can be selected.
However, the number of beam directions selected will generally be
much less than the number of possible beam directions which can be
selected.
In one modification of the present invention, the number of beams
to be selected can be varied. For example, a threshold could be set
so that only beams with a score of, for example, 6 or more would be
selected. This may have the advantage that more beams would be
selected when the mobile station is relatively close to the beam
transceiver station and less beams would be selected when the
mobile station is relatively far from the base transceiver station.
It is preferred that when the mobile station is located relatively
far from the BTS, that is greater than a critical distance, that a
relatively few beam directions be selected each beam having a
relatively high energy. However, when the distance between the BTS
and the mobile station is less than a critical distance, it is
preferred that a relatively large number of beam directions be
selected with each beam having a relatively low energy. The
critical distance is dependent on the environment of each
individual cell and may be around 0.5 to 1 km. The weighting
pattern or weighting algorithm may be such that the number of beams
which are selected is variable and that more beams are selected
when the mobile station is relatively close to the BTS.
Any suitable method can be used to determine whether or not the
distance between the MS and the BTS is greater than the critical
distance. In one embodiment, the channel impulse response obtained
for each of the possible directions is compared. If most of the
received energy is distributed in three or less beam directions,
then it is assumed than the distance between the BTS and MS is
greater than the critical distance. Alternatively, if most of the
received energy is received from four or more beam directions, then
it is assumed that the distance between the MS and BTS is less than
the critical distance.
It is also possible for a comparison block to use the timing
advance information in order to determine whether or not the
distance between the MS and BTS is greater or less than the
critical distance. This method is preferred in some embodiments of
the invention as it is gives more accurate results than the
previously outlined above.
Once a determination has been made as to the distance between the
mobile station and the BTS, this information can be used to select
a suitable weighting pattern or algorithm. For example, when the
BTS and the mobile station are relatively close, the weighting
pattern illustrated in FIGS. 5c or d may be selected.
In another modification to the present invention, a single weight
pattern may be stored in the weight pattern bank and that fixed
pattern is used for all situations.
It should be emphasised that any suitable weighting pattern can be
used and not just the weighting patterns shown in FIGS. 5a to d.
The weighting patterns illustrated can be replaced by suitable
algorithms which calculate appropriate weights. Such algorithms can
take into account various factors such as the radio environment and
the distance of the mobile station and the base transceiver station
in order to calculate the appropriate weights. The weighting
patterns can differ for different beam directions or the same for
all beam directions.
In another modification to the present invention, not only is beam
direction and signal strength information determined and stored by
the beam pre-selection and memory block 100 and 102, but also the
weighting patterns are applied to the information on the beam
strength by the weighting pattern block to determine the strength
of the data burst to be transmitted in the or each of the beam
directions. The power for the data burst to be transmitted in the
selected beam direction or directions will then be determined based
on the strength of the data bursts received on the K preceding data
bursts. The strength which is determined may differ for the
different beam directions when more than one beam direction is
selected.
It should be appreciated that the weighting pattern applied by the
spatio-temporal weighting pattern block may be changed on a burst
by burst basis.
Reference will now be made to FIG. 6 which schematically
illustrates in more detail the digital signal processor 21. It
should be appreciated that the various blocks illustrated in FIG. 6
do not necessarily correspond to separate elements of an actual
digital signal processor 21 embodying the present invention. In
particular, the various blocks illustrated in FIG. 6 correspond to
various functions carried out by the digital signal processor 21.
In one embodiment of the present invention, the digital signal
processor 21 is at least partially implemented in integrated
circuitry and several functions may be carried out by the same
element.
Each signal received by the digital signal processor 21 on the
respective inputs 19a-h is input to a respective channel impulse
response (CIR) estimator block 30. The CIR estimator block 30
includes memory capacity in which the estimated channel impulse
response is stored. The CIR estimator block also includes memory
capacity for temporarily storing the received signal. The channel
impulse response estimator block 30 is arranged to estimate the
channel impulse response of the channel of the respective input
19a-h. As already discussed an associated channel can be defined
for the given data burst transmitted in the selected frequency
band, the allocated time slot and the beam direction from which the
signal is received. The beam direction from which a signal is
received is ascertained by the Butler matrix circuitry 8 so that a
signal received at input 19a of the digital signal processor
represents mainly the signal that has been received from the
direction of beam b.sub.1 and so on. It should be appreciated that
the signal received at a given input may also include the side
lobes of the signal received on, for example, adjacent inputs.
Each data burst which is transmitted from a mobile station MS to
the BTS 4 includes a training sequence TS. However, the training
sequence TS.sub.RX which is received by the BTS 4 is affected due
to noise and also due to multipath effects which leads to
interference between adjacent bits of the training sequence. This
latter interference is known as intersymbol interference. TS.sub.RX
is also affected by interference from other mobile stations, for
example mobile stations located in other cells or cell sectors
using the same frequency which may cause co-channel interference.
As will be appreciated, a given signal from the MS may follow more
than one path to reach the BTS and more than one version of the
given signal may be detected by the antenna array 6 from a given
direction. The training sequence TS.sub.RX which is received from
input 19a is cross correlated by the CIR estimator block 30 with a
reference training sequence TS.sub.REF stored in a data store 32.
The reference training sequence TS.sub.REF is the same as the
training sequence which is initially transmitted by the mobile
station. In practice the received training sequence TS.sub.RX is a
signal modulated onto a carrier frequency while the reference
training sequence TS.sub.REF is stored as a bit sequence in the
data store 32. Accordingly, before the cross-correlation is carried
out, the stored reference training sequence is similarly modulated.
In other words the distorted training sequence received by the BTS
4 is correlated with the undistorted version of the training
sequence. In an alternative embodiment of the invention, the
received training sequence is demodulated prior to its correlation
with the reference training sequence. In this case, the reference
training sequence would again have the same form as the received
training sequence. In other words, the reference training sequence
is not modulated.
The reference training sequence TS.sub.REF and the received
training sequence TS.sub.RX each are of length L corresponding to L
bits of data and may for example be 26 bits. The exact location of
the received training sequence TS.sub.RX within the allocated time
slot may be uncertain. This is because the distance of the mobile
station MS from the BTS 4 will influence the position of the data
burst sent by the MS within the allotted time slot. For example, if
a mobile station MS is relatively far from the BTS 4, the training
sequence may occur later in the allotted time slot as compared to
the situation where the mobile station MS is close to the BTS
4.
To take into account the uncertainty of the position of the
received training sequence TS.sub.RX within the allotted time slot,
the received training sequence TS.sub.RX is correlated with the
reference training sequence TS.sub.REF n times. Typically, n may be
for example 7 or 9. It is preferred that n be an odd number. The n
correlations will typically be on either side of the maximum
obtained correlation. The relative position of the received
training sequence TS.sub.RX with respect to the reference training
sequence TS.sub.REF is shifted by one position between each
successive correlation. Each position is equivalent to one bit in
the training sequence and represents one delay segment. Each single
correlation of the received training sequence TS.sub.RX with the
reference training sequence TS.sub.REF gives rise to a tap which is
representative of the channel impulse response for that
correlation. The n separate correlations gives rise to a tap
sequence having n values.
Reference is now made to FIG. 7 which shows the channel impulse
response for four of the eight possible channels corresponding to
the eight spacial directions. In other words, FIG. 5 shows the
channel impulse response for four channels corresponding to a given
data burst received in four of the eight beam directions from the
mobile station, the data burst being in a given frequency band and
in a given time slot. The x axis of each of the graphs is a measure
of time delay whilst the y axis is a measure of relative power.
Each of the lines (or taps) marked on the graph represents the
multipath signal received corresponding to a given correlation
delay. Each graph will have n lines or taps, with one tap
corresponding to each correlation.
From the estimated channel impulse response, it is possible to
determine the location of the training sequence within the allotted
time slot. The largest tap values will be obtained when the best
correlation between the received training sequence TS.sub.RX and
the reference training sequence TS.sub.REF is achieved.
The CIR estimator block 30 also determines for each channel the
five (or any other suitable number) consecutive taps which give the
maximum energy. The maximum energy for a given channel is
calculated as follows: ##EQU2##
where h represents the tap amplitude resulting from a cross
correlation of the reference training sequence TS.sub.REF with the
received training sequence TS.sub.RX. The CIR estimator block 30
estimates the maximum energy for a given channel by using a sliding
window technique. In other words, the CIR estimator block 30
considers each of five adjacent values and calculates the energy
from those five values. The five adjacent values giving the maximum
energy are selected as representative of the impulse response of
that channel.
The energy can be regarded as being a measure of the strength of
the desired signal from a given MS received by the BTS 4 from a
given direction. This process is carried out for each of the eight
channels which represent the eight different directions from which
the same data burst could be received. The signal which is received
with the maximum energy has followed a path which provides the
minimum attenuation of that signal.
An analysis block 34 is provided which stores the maximum energy
calculated by the CIR estimator block 30 for the respective channel
for the five adjacent values selected by the CIR estimator block as
being representative of the channel impulse response. The analysis
block 34 may also analyse the channel impulse responses determined
by the CIR block 30 to ascertain the minimum delay. The delay is a
measure of the position of the received training sequence TS.sub.RX
in the allotted time slot and hence is a relative measure of the
distance travelled by a signal between the mobile station and the
BTS 4. The channel with the minimum delay has the signal which has
travelled the shortest distance. This shortest distance may in
certain cases represent the line of sight path between the mobile
station MS and the BTS 4.
The analysis block 34 is arranged to determine the position of the
beginning of the window defining the five values providing the
maximum energy. The time delay is then determined based on the time
between a reference point and the beginning of the window. That
reference point may be the common time when all received training
sequences in each branch start to be correlated, the time
corresponding to the earliest window edge of all the branches or a
similar common point. In order to accurately compare the various
delays of the different channels, a common timing scale is adopted
which relies on the synchronisation signal provided by the BTS 4 in
order to control the TDMA mode of operation. In other words, the
position of the received training sequence TS.sub.RX in the
allotted time slot is a measure of the time delay. It should be
appreciated that in known GSM systems, the delay for a given
channel is calculated in order to provide timing advance
information. Timing advance information is used to ensure that a
signal transmitted by the mobile station to the BTS falls within
its allotted time slot. The timing advance information can be
determined based on the calculated relative delay and the current
timing advance information. If the mobile station MS is far from
the base station, then the MS will be instructed by the BTS to send
its data burst earlier than if the mobile station MS is close to
the BTS.
The results of the analysis carried out by each of the analysis
blocks 34 are input to the beam selection block 101 which has
already been described in relation to FIG. 4.
The beam pre-selection block 100 of the beam selection block 101
uses the estimated channel impulse response in order to make the
beam pre-selection. There are a number of different ways in which
this can be achieved. If, for example, the beam pre-selection block
100 is to determine a single beam direction for a given burst, then
the beam pre-selection block 100 may ascertain which channel and
hence which beam direction has the desired maximum energy for a
given data burst in a given frequency band in a given time slot.
This means that the beam direction from which the strongest version
of the given data burst is received can be ascertained. This
direction may be used as the selected beam direction.
Alternatively, the beam pre-selection block 100 may ascertain which
of the channels has a minimum delay. In other words, the channel
and hence the beam direction having the data burst which has
followed the shortest path can be ascertained and used as the
selected beam direction for a given data burst.
It should be appreciated that in embodiments of the present
invention, more than one beam direction can be selected by the beam
pre-selection block 100 for a given data burst. For example, the
two directions from which the strongest version of a given data
signal are received can be selected as the given beam directions.
Likewise, the two beam directions providing the signal with the
least delay may be selected as the beam directions. It would of
course be possible for the beam pre-selection block 100 to
ascertain the direction from which the strongest signal is received
as well as the direction having the least delay and selecting those
two directions as the selected directions.
The beam pre-selection block 100 may also receive the associated
energy, calculated by the respective analysis block 34, for the
given selected beam direction.
The beam selection block 100 provides an output to generating block
38 which indicates which beam directions are to be used to transmit
signals from the BTS 4 to the MS and also the appropriate power
level to be used with each of those beam directions.
Generating block 38 is responsible for generating the signals which
are to be output from the digital signal processor 21. The
generating block 38 has an input 40 representative of the speech
and/or information to be transmitted to the mobile station MS.
Generating block 38 is responsible for encoding the speech or
information to be sent to the mobile station MS and includes a
training sequence and a synchronising sequence within the signals.
Block 38 is also responsible for production of the modulating
signals. Based on the generated signal and determined beam
direction, generating block 38 provides signals on the respective
outputs 22a-h of the digital signal processor 21. The generating
block 38 also provides an output 50 which is used to control the
amplification provided by amplifiers 24 to ensure that the signals
transmitted in the principal one or more beam directions have the
required power levels.
The output of the channel impulse response block 30 is also used to
equalise and match the signals received from the mobile station MS.
In particular, the effects of intersymbol interference resulting
from multipath propagation can be removed or alleviated from the
received signal by the matched filter (MF) and equaliser block 42.
It should be appreciated that the matched filter (MF) and equalizer
block 42 has an input (not shown) to receive the received signal
from the MS. The output of each block 42 is received by recovery
block 44 which is responsible for recovering the speech and/or the
information sent by the MS. The steps carried out by the recovery
block include demodulating and decoding the signal. The recovered
speech or information is output on output 48.
It should be appreciated that whilst the above described embodiment
has been implemented in a GSM cellular communication network, it is
possible that the present invention can be used with other digital
cellular communication networks as well as analogue cellular
networks. The above described embodiment uses a phased array having
eight elements. It is of course possible for the array to have any
number of elements. Alternatively, the phased array could be
replaced by discrete directional antennae each of which radiates a
beam in a given direction. The Butler matrix circuitry can be
replaced by any other suitable phase shifting circuitry, where such
circuitry is required. The Butler matrix circuitry is an analogue
beam former. It is of course possible to use a digital beam former
DBF or any other suitable type of analogue beam former. The array
may be controlled to produce more than eight beams, even if only
eight elements are provided, depending on the signals supplied to
those elements.
It is also possible for a plurality of phased arrays to be
provided. The phased arrays may provide a different number of
beams. When a wide angular spread is required, the array having the
lower number of elements is used and when a relatively narrow beam
is required, the array having the larger number of elements is
used.
As will be appreciated, the above embodiment has been described as
providing eight outputs from the Butler matrix circuitry. It should
be appreciated that in practice a number of different channels will
be output on each output of the Butler matrix at the same time.
Those channels may be different frequency bands. The channels for
different time slots will also be provided on the respective
outputs. Whilst individual amplifiers, processors, analogue to
digital converters and digital to analogue converters have been
shown, these in practice may be each provided by a single element
which has a plurality of inputs and outputs.
It should be appreciated that embodiments of the present invention
have applications other than just in cellular communication
networks. For example, embodiments of the present invention may be
used in any environment which requires directional radio
communication. For example, this technique may be used in PMR
(Private Radio Networks) or the like.
* * * * *